The main aim of this research project is to characterize in detail the ionic fluxes and the current-voltage relation of the potassium inward rectifier of striated muscle. This rectifier will be studied for values of external potassium concentrations near normal (i.e., 0-10 mM) both in sodium containing and sodium free solutions. The modifications produced by various agents blocking this potassium conductance system (i.e., Barium, quaternary ammonium ions, aminopyridines) will be also studied. The inward rectifier is located in the membranes of muscle covering the external surface and those in the transverse tubular system. It is the major ionic pathway controlling the membrane potential and other electrical properties of resting muscle. The properties of this rectifier are probably among the chief factors involved in hypotension and skeletal muscle weakness associated with hypokalemia and the paralysis of skeletal muscle associated with hyperkalemia.